Recent observations in this laboratory have indicated that endogenous retroviral envelope antigen is expressed in plasma cells of chickens possessing line 15I(5) genetic information. Our aim in this grant is to further characterize this phenemenon. In particular, we propose to i) characterize biochemically the viral-related, plasma cell-associated antigen and ii) to investigate possible immunological ramifications of viral envelope antigen expression in B cells. In terms of the latter proposal, the hypothesis that viral envelope antigen expression in plasma cells may have relevance for humoral immune responsiveness derives from observation that 15I(5) x 7(2) chickens infected with exogenous avian tumor virus produce type E-specific antibody (E-reactive antibody) that recognizes endogenous viral glycoprotein. The possibility is therefore suggested that this antibody interacts with the viral envelope antigen expressed on the surface of plasma or preplasma cells, an interaction which results in cell death or inactivation. Experiments are proposed to i) compare parameters of humoral immune responsiveness in viral envelope antigen positive and negative chickens and ii) assay directly for immunosuppression mediated by E-reactive antibody.